The present invention relates to accelerometers used in active suspension systems for vehicles, and in particular to their calibration.
It is known, for example as discussed in U.S. Pat. No. 5,742,919, that accelerometers used for controlling vehicle suspensions are subject to drift and other sources of inaccuracy, and that they need to be calibrated or checked in some way. The method proposed for this in U.S. Pat. No. 5,742,919 is to use different sensors on the vehicle to measure the same parameter, and thereby check that the measured values are correct. However, this requires there to be a large number of sensors and a certain amount of duplication of measurements.
The present invention provides a vehicle suspension system including a control means arranged to respond to signals from a lateral accelerometer arranged to measure lateral acceleration of the vehicle characterized in that the control means is arranged to measure an average of the signal from the accelerometer over time to determine a reference signal which corresponds to zero lateral acceleration, and to compare the instantaneous signal with the reference signal to measure the instantaneous lateral acceleration.
Preferably the control means is arranged to continually update the average during use of the vehicle. This ensures that any drift in the accelerometer is compensated for.
For example the average may be produced by integrating over time the value of the signal from the accelerometer.
Preferably the average is a weighted average. For example weighting factor may be used which preferably dependent on the instantaneous speed of the vehicle is arranged to bias the averaging towards signals produced when the lateral acceleration is low. Preferably the weighting factor is greatest at a predetermined vehicle speed and falls off at speeds higher than said predetermined speed and at speeds lower than said predetermined speed.
Preferably signals corresponding to lateral accelerations above a predetermined maximum value, which may vary with the speed of the vehicle, and is preferably zero for low vehicle speeds, are not included in the averaging process.